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The concept is simply to focus light in order to boost electricity output. But there's a wide disparity in the types of solar concentrators being built, from utility-scale solar thermal projects to specialized photovoltaic solar panels that could one day go on a homeowner's roof.
In this FAQ, we will specifically discuss concentrating photovoltaics, a design being pursued by a number of solar companies seeking to lower the cost per kilowatt the sun can deliver.
What are the primary forms of solar concentrators?
Solar concentrators use lenses, mirrors, parabolic dishes or other optics to concentrate energy from the sun. Very often, they have a mechanism so that these devices track the path of the sun during the day. In solar thermal applications, troughs or large mirrors amplify sunlight to create heat, which heats a liquid or gas that turns turbines to make electricity. Solar thermal is used for large-scale power plants operated by utilities, usually in the desert. After a 16-year hiatus, companies are opening up new plants or contemplating new ones in the southwestern U.S., India, southern Europe and North Africa.
This same technique is also being pursued in conjunction with photovoltaic solar cells, which convert light to electricity. Among concentrating photovoltaic companies, there is a wide range of approaches. There are systems designed for utilities' central power stations, mounted concentrators that can go on the roof of an office building, and those that are the same size as traditional solar panels.
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Why is there interest in concentrating photovoltaics?
Three words: the solar constant. The sun radiates about a kilowatt of energy per square meter on the surface of Earth, according to B.J. Stanberry, CEO of HelioVolt. There are 2.6 million square meters in a square mile. Thus, every square mile gets about 2.6 gigawatts. It's a number that just can't be increased.
Concentrators essentially try to artificially increase the constant by virtually expanding the size of solar cell with mirrors or lenses. The quality of the concentrator is rated by how much solar real estate it can cram onto a solar cell without creating things like shadows or interfering with other solar cells.
One number you hear a lot is how many suns a concentrator replicates. GreenVolts, which is commercializing technology licensed from Lawrence Livermore National Laboratory, has a concentrator it says can deliver the equivalent of the energy of 625 suns to a solar cell.
Why not just improve solar cells?
That's also being done. Without concentration, the efficiency of commonly used solar cells made from silicon tops out at around 22 percent. Physics says that crystalline silicon PV cells will top out at around 29 percent.
High-efficiency cells, historically used for satellites or spacecraft and made from different layers of materials, can exceed 40 percent efficiency or more, but this pushes up the price. Focusing more light onto cells makes them more productive.
The relatively high cost of photovoltaic material--the most common being silicon, which is in short supply--represents a significant cost to an overall solar power device. Using concentration, manufacturers are looking to lower the overall cost per kilowatt-hour of a solar power purchase. People often believe that since most solar cells are made of silicon, panel manufacturers inherit Moore's Law, which stipulates that the performance of microprocessors double every 24 months. But the same dynamics don't play out, solar industry executives say. Instead, the solar industry is focused exclusively on cost and making solar power competitive with traditional fossil fuel-based power generation. That's why many companies, including a number of start-ups, are trying to concentrate solar power, along with thin-film solar cells made from other materials, and lowering their manufacturing costs.
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Often, in the city where the piece of land is prime real estate, you are limited with the amount of space for sunlight capture. If you are intercepting the sunlight that would have otherwise fallen into other areas, you are in fact stealing solar energy from them and diverting it for your own solar cells.
What the net effect of PV solar concentration is to increase the energy output per unit amount of silicon used, and in no way in Physics you can call this as improvement of solar conversion efficiency. The perceived increased is only for marketing purpose, and to declare that there is an overall increase of efficiency is very misleading.
The real inquiry or the bottom line then would be to know if by concentrating the solar beams, will it result to lesser cost of production per unit energy output per unit area of land.
This leads to the next question of whether it makes more sense to do solar in a distributed (roof by roof) or centralized model. Much of the legislation seems to be encouraging distributed energy production - large subsidies are available to residents who put solar on their roofs. Technologies like concentrators just don't really apply here. The cost goes way up, and the extra weight and equipment may be impossible to mount on most roofs.
The biggest barrier to widespread residential solar is information. Most people find this topic so complex that they just don't bother, despite having high interest in being 'green'. Sites like www.solar4sf.org help in cutting through the complexity to make the information accessible to non-solar geeks.
The true barrier to solar is cheap panels and accessories. Also I'm gonna take a gamble and say that until hydrogen fuel cells replace batteries, there is no hope of people adopting solar power widely.
Installation costs are too high also. What we need is a cheap (under $3000 in current 2007 US Dollars) kit from WalMart that includes everything you need to get power from the Sun and could be easily self-installed....
http://pctechedu.blogspot.com/